The present invention relates to a camera that projects a pattern of visible light onto an object to be photographed to indicate to a photographer the area of the object within view of the camera.
In recent years, document scanners have become commonplace. Although these work well and are relatively inexpensive, a document scanner occupies a significant amount of scarce desk space.
The use of a camera to take a photograph of a document consisting of text and/or images offers one way of dealing with the problem of wasted desk space. In conventional film or electronic photography, a person may look through a viewfinder to aim the camera at an object. When the object is a document, for example a document lying on a desk, this means a person has to stand up, lean over the document, look through the viewfinder and then take the photograph. This is not very convenient. Furthermore, if the document is at the back of a desk, the person may have to move the document. In addition, the person""s body may block ambient light, thereby requiring the use of a flash. The use of a flash in an office environment may be unacceptable.
Recently, some digital cameras have been provided with a liquid crystal display (LCD) viewfinder that may be viewed at some distance and that can be oriented by a user so that the LCD viewfinder may be viewed over a range of angles, for example at right angles to the optical axis of the camera. By pointing the lens downward while keeping the viewfinder at a convenient angle, it would then be possible to take a picture of a document on a desktop from a sitting position. Unfortunately, it is not intuitively obvious to most people which way a hand-held camera should be moved by looking at the LCD viewfinder.
It is an object of the present invention to provide a camera that is more convenient to use when taking a photograph, for example a photograph of a document lying on a desk.
According to the invention, there is provided a camera comprising image capture means, a lens arranged to image an object in an object plane onto the image capture means, a light projector operable to project visible optical radiation onto the object plane to indicate to a user of the camera the field of view imaged onto the image capture means, and a camera attitude sensor system for sensing the attitude of the camera, wherein the image framing projector is operable in response to the attitude sensor system to project the optical radiation onto the object plane only when the attitude sensor system senses that the camera is oriented so that the optical radiation will be projected downwards.
The light projector may project just one area or pattern of light onto the object plane, for example a cross pattern centered in the camera field of view, or a box surrounding the field of view. Alternatively, more than one such area or pattern may be projected, for example four light beams which indicate the corners of the area in the object plane imaged by the camera.
Various light patterns, for example a cross, a framing box or four L-shaped framing indicators, may be formed in various ways. For example, the light source or sources may have corresponding shapes that are directed or focussed onto the image plane. Alternatively light from a point image source or sources can be shaped by passing the light through a transmissive optical diffractive element or reflecting the light off a reflective diffractive optical element.
In one embodiment of the invention, the light projector is an image framing projector operable to project visible optical radiation onto the object plane to indicate to a user of the camera the bounds of the object imaged onto the image capture means.
The camera can then be configured so that the optical radiation can projected either automatically or manually only when the optical radiation will be projected downwards.
Because the document will normally be below the level of the camera, for example lying on a desktop, the projected optical radiation can therefore be made available only when the camera is to be used to capture an image of such a document. This reduces the concern of such optical radiation being directed into a person""s eyes, which makes the camera potentially safer, as well as less likely to annoy such persons.
The term xe2x80x9clensxe2x80x9d as used herein is not restricted to a single lens element and includes lenses with compound optical elements.
The image capture means may be photographic film, or an electronic device, for example a detector array, in particular a two-dimensional CCD array or CMOS array.
The source of visible optical radiation is preferably a compact solid state device or devices, for example one or more light emitting diodes or laser diodes. There may be several such devices, for example, four devices each providing light projected onto one of the four corners of the object plane. Alternatively, one such device may be used with the light being split into discrete sources by a number of fibre optic light guides.
The attitude sensor system may comprise one or more tilt sensors or accelerometers, arranged to provide a signal indicative of the attitude of the camera.
So that the projected optical radiation is not projected when such radiation would be close to horizontal, it is preferred if the image framing projector is operable in response to the attitude sensor system to project the optical radiation onto the object plane only when the attitude sensor system senses that the camera is oriented so that the optical radiation will be projected downwards with a predetermined angle at least 30xc2x0 below horizontal. Such as angle corresponds with that which may be appropriate when using a hand-held camera to image a document lying on a work surface such as a desktop. A further degree of safety can be achieved if the optical radiation is projected onto the object plane only when the attitude sensor system senses that the camera is oriented so that the optical radiation is projected downwards within 30xc2x0 of vertical. This may be appropriate if the camera is mounted on a support nearly above a document to be imaged, such as a post attached to the side of a work surface or desktop.
Optionally, the camera may have means by which the angle at which the projected light becomes operable can be selected by a user of the camera.
In a preferred embodiment of the invention, the attitude sensor system senses the rate of change of the attitude of the camera, the image framing projector being operable in response to the attitude sensor system to project the optical radiation onto the object plane only when the attitude sensor system senses that the rate of change of camera attitude is below a predetermined threshold.
For the convenience of the user, the camera may have a timer that times a predetermined time delay in response to the camera attitude sensor when the camera has been oriented so that the optical radiation would be projected downwards. The optical radiation is then automatically projected downwards following the predetermined time delay.
It may be, however, that a person may wish to image a document that is not below the level of the camera. For example, if the document is posted on a notice board, then it may even be above the level of the camera. Therefore, the camera may have an override feature, such as a user-selectable switch, that disables the operation of the camera attitude sensor system.
Preferably, however, the camera has two or several automatically configurable modes of operation, for example in addition to a document-capture mode, the camera have a scene-capture mode, such as a still camera mode, or video camera mode. In this case, the camera is automatically configured in a document-capture mode when the attitude sensor system senses that the camera is oriented so that the optical radiation will be projected downwards. The camera may then be automatically configured in a different mode of operation when the attitude sensor system senses a different camera attitude in which the optical radiation would not be so projected downwards.
Since cameras generally have a useful depth of focus, the image need not lie exactly in focus. For example, when taking a picture from a seated position of a document lying on a desktop, the document will generally be at an angle to the exact object plane. However, as long as the document lies within the depth of focus of the camera, it may still be imaged accurately. Therefore, the term xe2x80x9cobject planexe2x80x9d as used herein is not limited to the exact object plane, but includes other object planes within the depth of focus of the camera.
A user may therefore use the camera, without the need to peer through any viewfinder, to point roughly in the direction of the object, for example a document lying on a desktop, and then move the camera until the projected visible optical radiation indicates the bounds of the camera""s view, whereupon the picture may be taken.
If the camera is not hand-held, but mounted, for example on a bracket attached to a desktop or other item of office furniture, then the document may be moved until it falls within the bounds indicated by the projected optical radiation.
In one embodiment of the invention, at least two points, lines or areas of optical radiation are projected onto the object plane, for example at opposite corners of a rectangular field of view for the camera. Preferably, there is a pattern of projected optical radiation that indicates all four corners of a camera""s field of view. Such a pattern may then bracket or delineate the bounds of the object to be imaged onto the image capture means.
In one embodiment of the invention, the source of optical radiation is projected onto the object plane independently of the lens. The source may therefore have optical elements to collimate or focus the optical radiation. If the source is a commercially available laser diode, then these elements may be included with the packaging of the laser diode device. The beams of optical radiation from the sources may then be angled to correspond with the periphery of the angle of view of the lens.
In an alternative embodiment of the invention, the lens projects the optical radiation onto the object plane. The sources of optical radiation may therefore be provided within the protective confines of a camera body.
The camera may have a viewfinder and a mirror that reflects light between the lens and viewfinder. Optical radiation is then directed to the lens via a reflection from the mirror.
If the camera is of the single lens reflex type, then the mirror may be movable. This arrangement is particularly suitable in the case that the image capture means is photographic film, because then it is possible to design the camera to keep stray light from the sources from reaching and exposing the film. Alternatively, the mirror may be fixed and partially reflective and transmissive.
For applications such as imaging of a document on a desktop, it is preferable if the image capture means is an electronic detector array. In this case, the source of optical radiation may be provided within the camera in an image plane of the lens. Stray light from the sources may still, of course, affect such a detector array, in which case the sources or sources of optical radiation may be switched off just before the detector array, and any associated electronics, are activated to capture the image. Because the image capture time of such an array is quite fast, of the order to 50 ms or less, a momentary disappearance of the visible optical radiation may scarcely be noticed by the user of the camera. Therefore, no optical radiation may be projected when the image capture means captures an image.
In order to keep the pattern of projected optical radiation well defined across a range of camera operating distances, it is advantageous if the camera has means for focussing in conjunction both an image of the object on the image capture means and the optical radiation projected onto the object plane. In the case of a source of optical radiation inside the camera that passes through the lens, this may be done by focussing the lens or moving the image capture means and the optical source or sources to lie on an in-focus image plane. The optical radiation projected onto the object may therefore be kept in focus automatically as the camera focuses on the object.